Using a private investigator should be an exception rather than the rule. This article I read gives food for thought about the circumstances in which the instruction of a private investigator might be useful. 

In financial remedy matters in particular, if you have cause to believe that the other party is not being truthful about their cohabitation status, assets or employment, the instruction of an investigator may gather important and critical information needed within the court proceedings. Other expects could be instructed simultaneously or consequentially such as a forensic accountant or business expert. 

In Children Act proceedings, caution will need to be used as we will need to weigh the importance of gathering information versus the invasion of privacy and the potential to have allegations of control / other domestic abuse being levied at the instructing parent. 

However, for example, an investigator could be useful where there are concerns about a parent's involvement with a third party. If that parent is stating to the court that they are no longer involved or associating with that third party and there is doubt as to the parent's truthfulness, then an investigator could be a useful way to either provide reassurance or to prove that the parent is being untruthful to the court. We would want to consider the facts of each matter and make decisions accordingly, with parents being reminded that the instruction of an investigator will not be the default.